The present invention relates to a current ratio-differential relay which responds to the ratio of an operating force in proportion to a differential current as a result of vector summation of currents of all lines connected to the bus of a power system, to a suppressing force based on the maximum current of the lines, and more particularly, to a relay means which is devised for the saturation of current transformers.
In FIG. 1, choosing the polarity of currents I.sub.Fl -I.sub.Fn flowing from lines L.sub.l -L.sub.n into the bus to be positive, it is known that an internal fault shown in FIG. 1(a) causes the total current ##EQU1## to be unequal to zero, while an external fault shown in FIG. 1(b) causes the total current to equal zero. Based on this nature, the conventional current ratio-differential relay has been designed to discriminate an internal fault and external fault from the magnitude of the total current I.sub.F. Such an arrangement of the relay is disclosed in Japanese Patent Publication No. 12571/75. A catalog of BROWN BOVERI, entitled Electronic Busbar Protection (based on directional comparison), No. CH-ES36-10E, discloses the technology for compensating the effect of the saturation of current transformers through the extension of the signal duration for the input larger than the setup value. An input of only 1 ms due to the saturation of the current transformer is extended to form a pulse of 4 ms for use in directional comparison. This technology, however, is for the directional comparison only, and does not provide the ratio-differential function.
At the occurrence of an external fault, the currents concentrate into outflow lines, leaving the inflow currents depending on the number of power terminals and the power capacity, causing the large outflow currents to saturate the current transformers, and this results in a significant error in the differential current and eventually in a down-grade accuracy of the relay.
There has been proposed the arrangement of the current ratio-differential relay of this type coping with the saturation of current transformers, as shown in FIG. 2. For explanatory purposes, the following discussion views a power system of a bus-bar and only two lines, but the principle is applicable to systems with more lines merely by increasing the number of differential circuits 7 and inputs of the maximum value sampling circuit 8, as will be described in the following, corresponding to the number of lines.
The arrangement of FIG. 2 includes input terminals 1 and 2 of the relay for receiving the secondary currents I.sub.1 and I.sub.2 of current transformers CT1 and CT2, the first and second input transformers (TRFs) 3 and 4 for transforming the input signals at the input terminals 1 and 2 into values suitable for the relay, and the first and second rectifying circuits (RECTs) 5 and 6 for making full-wave rectification for AC signals. The rectified output signals are conducted to a maximum value detecting circuit (MAX) 8, which holds the maximum value of the input signal under a certain operating time constant and provides the output E.sub.T. The outputs of the input transformers 3 and 4 are also received by a differential circuit (DIF) 7 which evaluates the vector summation of currents in all lines (differential current I.sub.D) mentioned previously. The arrangement further includes the third rectifying circuit (RECT) 9 which makes full-wave rectification for the differential current and provides the output signal for a pair of first and second smoothing circuits (SMTs) 10 and 11. The second smoothing circuit 11 operates to smooth the input signal and multiplies it by .eta..sub.p to provide .eta..sub.p E.sub.p. A subtraction circuit (SUB) 12 calculates E.sub.T -.eta..sub.p E.sub.p and provides the output only when the resultant value is positive. A circuit 13 is the third smoothing circuit (SMT) with characteristics of fast charging and slow discharging, and provides the output signal E.sub.R together with the output signal Eo of the first smoothing circuit 10 to a comparison circuit (CMP) 14, which provides an active output if Eo-.eta..sub.R E.sub.R &gt;K, where Eo represents the operating force, E.sub.R is the suppressing force, .eta..sub.R is the suppressing ratio, and K is the setting value of the relay.
The operation of the relay will be described with reference to FIG. 3 on the assumption that the system is in internal fault with an outflow current from the bus-bar. The current transformer CT1 provides the secondary current I1 of the inflow current, while the CT2 provides the secondary current I.sub.2 of the outflow current (since the current flowing into the bus-bar is signed positive, the I.sub.2 has the opposite phase relationship with I.sub.1 and the magnitude is half that of I.sub.1). The differential circuit 7 produces the differential current I.sub.D which is proportional to the vector summation of currents I.sub.1 +I.sub.2. After the differential current I.sub.D has been rectified by the rectifying circuit 9, it is smoothed by the smoothing circuit 10 to obtain the differential output, that is, the operating force Eo. The output signals of the input transformers 3 and 4 are rectified by the first and second rectifying circuits 5 and 6, respectively, and their outputs are supplied to the sampling circuit 8, which in turn provides E.sub.T based on I.sub.1 (larger than I.sub.2 ). The output of the rectifying circuit 9 is smoothed by the smoothing circuit 11 and, at the same time, multiplied by .eta..sub.p to become .eta..sub.p E.sub.p. Here .eta..sub.p is an invariable coefficient determined from the characteristics of the relay. In this embodiment, .eta..sub.p is set to 2 for a reduced suppression force so that a sufficient operating force is obtained even in the presence of the outflow current half that in magnitude of the inflow current at the occurrence of internal faults. Accordingly, the subtraction circuit 12 provides E.sub.T -.eta..sub.p E.sub.p .ltoreq.0, causing the smoothing circuit 13 to provide no suppression force E.sub.R, and the comparison circuit 14 provides an active output, i.e., the relay responds to the fault. Namely, the suppression force is reduced so as to make the relay more sensitive for operation against internal faults.
FIG. 4 shows the waveform of output signals caused by the external fault in which a faulty current flows from the line on the terminal 1 through the bus-bar to the line on the terminal 2. The differential current I.sub.D is I.sub.1 +I.sub.2 =0, and the operating force Eo is not active. The value of E.sub.T reflecting the maximum value of the line currents becomes directly the suppression force E.sub.R =E.sub.T -.eta..sub.p E.sub.p. Accordingly, the comparison circuit 14 receives no operating force Eo, but instead, a large suppressing force E.sub.R, providing no active output, and thus the relay does not respond to the fault.
In the conventional bus protection relay, as described above, the output signals Eo and E.sub.R are processed in analog values before they are entered to the final comparison circuit 14 as shown in FIG. 2, and the formula of criterion is expressed as follows. EQU Eo-.eta..sub.R [E.sub.T -.eta..sub.p E.sub.p ]&gt;K (1)
In formula (1), E.sub.R is equal to E.sub.T -.eta..sub.p E.sub.p, and the comparison circuit 14 is effective only when this value is positive. FIG. 5 shows the case where the input signals I.sub.1 and I.sub.2 include respectively a DC component, and there are two cases: the input signal I.sub.2 having the same level as of I.sub.1, and the I.sub.2 having a distorted chopped waveform. In the latter case, the differential current appears as shown by I.sub.D in FIG. 5. This causes the smoothing circuit 10 to produce the differential output Eo. However, the output E.sub.T from the circuit 8 is based on I.sub.1, the E.sub.T rises faster than the differential output Eo, i.e., .eta..sub.p E.sub.p, resulting in a sharp rise of the suppression force E.sub.R, whereby the output Eo can be suppressed.
In case .eta..sub.p E.sub.p exceeds the output E.sub.T of the circuit 8 momentarily, the suppressing force E.sub.R does not fall instantaneously since the smoothing circuit 13 is connected at the following stage, and the output E.sub.R of the smoothing circuit 13 takes only positive value. However, if the output signal .eta..sub.p E.sub.p of the smoothing circuit 11 increases or extends in time due to the saturation, the analog processing may not be able to respond, resulting possibly in a failure of operation.
The conventional current ratio-differential relay arranged as described above is apt to malfunction due to the saturation of current transformers, and moreover, it needs complicated adjustments for setting the analog waveforms.